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Solar and terrestrial radiation and absorption

Watch Dr. Nathan B. English disuss solar and terrestrial radiation and absorbtion
<v ->Welcome to step 4.</v> In this step,we’re going to talk about solar and terrestrial radiation and atmospheric absorption of that radiation. And this is really the crux of the greenhouse gas argument.
Now, remember in the last step, we looked at the differences in incoming solar radiation and outgoing terrestrial radiation. And that incoming solar radiation occurs in these very high frequencies and or short wavelengths. High-frequency short wavelength, electromagnetic radiation. When that radiation is absorbed by the Earth surface or the Earth’s atmosphere, it’s re-emitted out into space or from the Earth’s surface as sort of longer wavelength,
lower frequency, outgoing terrestrial radiation. So there’s those differences in wavelengths of the incoming and outgoing radiation. And this is really important because different gases in the atmosphere absorb electromagnetic radiation at different frequencies. Which means that light only penetrates and moves through different gases at different frequencies. And I’m going to use light sometimes interchangeably with electromagnetic radiation. And when I say light I mean electromagnetic radiation from sort of the infrared to the, to the UV spectrum. Cause that’s where I really want to focus our conversation. Now, what does that mean? It means that light, can..absorbed…
If you absorb 100% of the light with a gas, that means if I’m shining a laser through a gas that absorbs 100% of that frequency of laser light. If I’m looking at that laser from the other side of that gas, I won’t see any of the laser light. Now, all that energy in the laser light will be absorbed by that gas. So slowly over time, that gas will actually heat up because it’s having energy added to it by the laser.
Now, if I have a gas that’s transparent to the laser, and I shine a laser through a gas transparent to that particular frequency of light from the laser, I’ll definitely see it with my eyeball on the other side, probably burn my eyeball out, but that gas won’t heat up from that laser because all the energy in that laser light is passing through the gas. Likewise, with our atmosphere, solar radiation will pass through our atmosphere because there are different windows available for solar radiation to pass through our atmosphere. And when I say window, what I’m talking about is I’m talking about a… frequencies of light that aren’t absorbed by our atmosphere or aren’t absorbed by a particular gas.
So I’m talking about windows in the transparent sense, meaning it’s a portion of the electromagnetic spectrum that is not absorbed by that gas, meaning the light can travel freely through it. So let’s take methane, for example, which is a really powerful greenhouse gas, 30 times more powerful than CO2. It’s really good at absorbing light in the infrared spectrum. Okay, in the middle of the infrared spectrum. Now, any other light can travel right through it. So visible light and UV light are not affected by methane. They pass right through it, it’s a window.
In contrast, ozone and oxygen have a small
sort of absorption spectrum in the infrared frequencies. It’s not very big, it’s pretty narrow. But they have a huge absorptions spectrum here in UV light. And there’s a window everywhere in between. So visible and infrared light can easily pass through oxygen and ozone, but UV light is trapped pretty effectively by oxygen and ozone, meaning it’s absorbed and all that heat energy from that radiation is absorbed or all that energy from that UV radiation is absorbed by oxygen and ozone. And that’s why you actually see an increase in temperature in the stratospheres because that’s a reflection of the ozone absorbing that UV radiation and heating up in our atmosphere. Now, carbon dioxide has a complicated relationship.
it’s completely transparent to visible in UV light and a bit of, and quite a bit of infrared light, but it has these sort of absorption spectra here in the middle of the infrared spectrum and in the far left hand side of the infrared spectrum. I’m getting to the point here. This is important. Water vapor is a huge absorber of infrared radiation. If you want to hide from an infrared camera, find some water vapor and hope that your infrared emissions are somewhere in these absorption spectra and not on the windows.
Now, when you add all of these gases together in the proportions that there exist in the atmosphere, you get this sort of total absorption spectra for all gases in the atmosphere on the bottom here. And what’s important to notice here is not the peaks in the plateaus, but the valleys. These valleys are what we call the atmospheric windows. These are where radio electromagnetic radiation can travel freely through the atmosphere and then the peaks and plateaus are where radiation is absorbed. Now notice that sunlight has a great big open window and all the energy that sunlight delivers to Earth has a great big open window here. In the visible light spectrum.
So lots of energy is able to get through the atmosphere and impact on the surface of the Earth. When it impacts on the surface of the Earth, it’s absorbed by the Earth and the Earth warms up. Not to 6,000 ° K, but to about…what was it before? 200 and.. about 300° K.
…290°K. Okay, so that sunlight comes in, but remember that the Earth is re-emitting radiation at lower frequencies. Longer wavelengths, lower frequencies. So Earth’s radiation is re-emitting back into space through these very narrow windows in the infrared band. Now notice what’s sort of hanging around these infrared windows okay is methane , nitrous oxide, carbon dioxide, and water vapor, to a small extent. Now, if I increase the amount of methane or carbon dioxide in the atmosphere, what happens is these peaks tend to get a little bit wider and a little bit taller. And when you do that, what you’re doing is you’re actually, when you add it all up, you’re closing.
You’re making these atmospheric windows narrower and less radiation is able to escape the surface of the Earth. It’s intercepted by the atmosphere before it’s re-radiated from the top of the atmosphere. So essentially what happens is, as we release these trace gases, carbon dioxide and methane and nitrous oxide to a small extent, and as the temperature of Earth increases in water vapor more easily enters the atmosphere. We’re essentially closing these atmospheric windows. So sunlight checks in ,energy checks in, but energy can’t check out. So that’s that old Men in Black movie, “roaches check-in but roaches don’t check out.”
When we look at this in a sort of way back up here, now we’re looking at the impact of solar radiation on the Earth surface and the re-emission of infrared radiation from the Earth’s surface. We can start to think about where all that energy and heat are stored. Okay, remember, heat is a kind of energy.
And so ultraviolet..sorry, electromagnetic radiation is a kind of energy that can be converted into a different kind of energy, heat energy. Typically what we get at the top of the atmosphere is we about 340.4 Watts per meter squared of energy coming in from the sun. Remember, this is all the high frequency,
short wavelength light, very energetic, 6,000 ° K sun. And it’s impacting at the top of the atmosphere. A fraction of this about 160, about half of it reaches the Earth’s surface. The rest of it is reflected by clouds and atmosphere or reflected by ice and oceans on the Earth’s surface. So about a third of this is reflected back into space as this short wave radiation and about 163 of it is absorbed by the surface of the Earth and sorry, 77 W/m² is absorbed by the atmosphere itself. So, it doesn’t make it all the way through. There are some atmospheric windows that sunlight can’t travel through. So they get picked up in the 77 W/m².
Once that energy has hit Earth, it’s, re-emitted back to the atmosphere. It’s re-emitted from the Earth’s surface. And notice this has 398. Woaw, wait a minute, there’s only 163 coming in. It turns out that the, because of the atmospheric windows are pretty tight. The atmosphere actually reabsorbs a lot of that terrestrial radiation. And when it reabsorbs that terrestrial radiation, it actually emits energy itself. The atmosphere admit it, emits energy cause it’s mass and you can treat the atmosphere as a black body if you want. And that radiation from the atmosphere actually re-impacts on the Earth. See that over here, this back radiation, the add up three 40+163, and you get close to 398.
Now, and it’s going around and around and around. And don’t forget, we’ve got some light and heat over here, leaving and thermals and direction as well. Once the atmosphere is heated up by either the incoming solar radiation or the terrestrial radiation, then what happens is, the atmosphere can actually, radiate, re-radiate that energy back out into space. So total outgoing radiation is about 239.9. And if you add 240+23+77, you get pretty close to 340.
Now notice that this atmospheric window there’s a very small amount of radiation actually goes free from the terrestrial surface, straight out into space. All right, and the rest of it is emitted by the atmosphere or emitted by clouds. And there’s sort of this cycling here between terrestrial radiation and atmospheric back radiation. As you decrease these atmospheric windows, you’re essentially trapping more energy, more heat in this cycle of terrestrial radiation back radiation. And that’s what increases the temperature of the atmosphere. We’re always going to be, and as you increase the temperature of the atmosphere, you increase the frequency and the energy of the light of the electromagnetic radiation leaving from the top of the atmosphere.
So everything balances out, but essentially it’s this piggy bank of an atmosphere that you’re essentially by closing those atmospheric windows, you’re increasing the capacity and the ability of the atmosphere to contain heat and maintain heat. And that’s sort of where the greenhouse idea comes from, It’s that we’re essentially increasing the effectiveness of our atmosphere’s ability to attract heat and energy.

Now that we have discussed both the atmosphere and radiation, let’s look at how they interact with one another.

Watch Dr. Nathan B. English explain how solar radiation enters the atmosphere, the impact solar and terrestrial radiation have on the atmosphere and how greenhouse gases increase the effectiveness of our atmosphere’s ability to trap heat and energy.

Now it’s your turn

In this video, Dr. English compares the atmosphere to both a ‘piggy bank’ and a hotel in which sunlight checks in but it never checks out. Now that you have a clearer understanding of the science, what other comparisons could you use to explain the interaction between radiation and the atmosphere? Share your thoughts in the comments.

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